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 Table of Contents  
Year : 2022  |  Volume : 11  |  Issue : 1  |  Page : 5-9

Localization of L5–S1 disc space by utilizing simple on table surface marking in posterior lumbar spine surgery without any radiological assistance: An observational study

1 Assistant Professor, Department of Neurosurgery, Veer Surendra Sai Institute of Medical Science and Research, Burla, Odisha, India
2 Assistant Professor, Department of Orthopedics, Veer Surendra Sai Institute of Medical Science and Research, Burla, Odisha, India

Date of Submission13-Aug-2021
Date of Decision22-Oct-2021
Date of Acceptance03-Nov-2021
Date of Web Publication01-Feb-2022

Correspondence Address:
Tushar Ranjan Dalei
Quarter No. 3R/24 (Third Line), Doctor's Colony, Veer Surendra Sai Institute of Medical Science and Research, Burla, Sambalpur - 768 017, Odisha
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/NJCA.NJCA_104_21

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Background: Localization of the spinal levels is an important task during any spinal surgery. The purpose of the study is to assess the accuracy and reliability in identifying L5–S1 disc space by surface localization using Venus of dimple as a landmark in posterior spinal surgery without undertaking any radiological assistance. Methodology: We prospectively analyzed 39 patients, who had undergone primary posterior spine surgery, especially for Prolapsed intervertebral disc mostly affecting the lumbosacral region. Following surface marking using Venus of dimple as a landmark, patients were subjected to surgery with or without prior X-ray evaluation solely on surface localization of L5–S1 disc space. Intraoperatively, the accuracy and reliability of the surface marking were determined. Results: Sixty-four percent of the patients had accurate L5–S1 spine level localization utilizing the above clinical palpatory methods, while 23% of the cases identified L5 spine intraoperatively. On analyzing this surface topographical method with that of X-ray in standing position, the sensitivity and specificity came to be 93.3% and 49%, respectively, with a positive predictive value of 77.8%. Conclusion: The method of localization of L5–S1 space described although cannot fully replace the role of fluoroscopic assistance during spinal surgery, still it is an important, simple, and reliable alternative method of localization which demands its application by the spine surgeons, especially in the beginning of their carrier and in certain specific situations.

Keywords: Dimple of Venus, surface landmark, L5–S1 disc space

How to cite this article:
Pattajoshi AS, Dalei TR. Localization of L5–S1 disc space by utilizing simple on table surface marking in posterior lumbar spine surgery without any radiological assistance: An observational study. Natl J Clin Anat 2022;11:5-9

How to cite this URL:
Pattajoshi AS, Dalei TR. Localization of L5–S1 disc space by utilizing simple on table surface marking in posterior lumbar spine surgery without any radiological assistance: An observational study. Natl J Clin Anat [serial online] 2022 [cited 2022 Jul 1];11:5-9. Available from: http://www.njca.info/text.asp?2022/11/1/5/337038

  Introduction Top

Accurate identification of spinal levels is a prerequisite for the success and safety of any spinal surgery. Ability to consistently identify a spinal level not only improves the palpation reliability, but it also helps in patient care and treatment effectiveness. Radiographic identification of the spinal level is always more accurate than palpation of surface anatomical landmarks, however, the use of a radiograph for localization of the spinal axis not only exposes the patient radiation hazards but also imperative to operating room staffs.

Localization by palpation method using the intercristal line (Tuffier's line) is not always dependable though it crosses the spinal level with a maximal incidence at the L4 spinous process but varies from the L5–S1 interspace to the L3–L4 interspace.[1] This method is also limited by the technical difficulty associated with manually palpating the iliac crest.[2],[3] The accuracy and reproducibility of this method are usually affected by depth differences of subcutaneous adiposity or thickness of thoracolumbar musculature mostly seen in females and patients with higher body mass indices.[4] Similarly, palpation of the posterosuperior iliac spine (PSIS) although passes in between S1 and S2 vertebral foramen, still studies have yet to be carried out to confirm its reliability as a surface marker for determining spinal level in thoracolumbar surgery.[5] Wrong localization or failure to localize during surgery not only increases the anxiety of spine surgeons in the beginning of their carrier but also responsible for decreasing the confidence of patients over the treating surgeon, which may invite legal consequences in future. Thus, a spine surgeon should be well acquainted with the localizing methods in various ways to avoid such complications.

At present, there is a paucity of literatures regarding the prediction of accurate spine level localization in the posterior approach of lumbosacral spine surgery. This study is based on two objectives. The first one is to determine whether using the dimple of Venus as a surface landmark can predict L5–S1 disc space more accurately in a uniform manner. The second one is the applicability of this method in posterior spinal surgery without undertaking any radiological assistance. Overall, it introduces a novel localization method for posterior lumbosacral spine surgery.

  Materials and Methods Top

Study population

The study was conducted between 2019 April and 2020 October. A total 39 adult patients (24 males) undergoing primary posterior spine surgery, especially for prolapsed intervertebral disc (PIVD) affecting L4/L5 or L5–S1 space, were included in the study. Those having prior surgery at the same site, severe scoliosis, marked lumbar spinal deformity, or those with a lumbosacral area that was too painful for palpation were excluded from the study.

Description of the localization method

The sacral dimple of Venus (corresponds to the posterior superior iliac spine, also called beauty dimples) is marked on either side with a skin marker [Figure 1] when the patient is in prone position. These are the paramedian soft-tissue indentations in the region of the lateral sacrum to represent the location of PSIS [Figure 2].[6],[7] These two points are joined by a horizontal line (inter-dimple line). This line is generally over the proximal sacrum around S1 or S2 vertebra.[8] Another vertical line is drawn in the middle of the back (posterior median furrow) to join with this inter-dimple line. Two tangential lines are drawn from each dimple of Venus at a 15° angle to inter-dimple line toward the vertical line over the posterior median furrow. The point of intersection of these two angled lines with that of a vertical line corresponds to the L5–S1 space is marked with skin marker. The adjacent levels are delineated with this point as reference and surgical incision site is determined.
Figure 1: The method of localization corresponding to L5–S1 disc space taking Venus of dimple as a surface landmark

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Figure 2: Venus of dimple corresponds to posterior superior iliac spine (Source; Osteology Section of Anatomy Department of the institution with permission)

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All these localization methods were performed by two investigators and subsequently cross-checked by another observer to avoid inter-observer bias and to maintain the reliability and accuracy of the procedure.

Pre-operative X-ray imaging

A pre-operative lumbar spine X-ray imaging both in standing and supine position with a radiopaque marker attached to the anatomical landmarks to identify the L5-S1 disc space was undertaken in 22 patients selected as per convenience of the researchers. Accuracy of X-ray marking is compared with intraoperative findings. Lumbosacral anteroposterior view radiographs were examined by another investigator who was blinded to the localization method [Figure 3]. We analyzed these X-rays to see if there were any differences in the standing and supine postures of the patients.
Figure 3: Marker X-ray of a patient in standing posture corresponds to L5–S1 disc space (middle one)

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Surgical procedure

In all 39 patients, the L5-S1 space is identified completely on the regional topographical anatomical landmarks, and accuracy was checked intraoperatively. All patients underwent discectomy. Intraoperative disc space was confirmed either by extending the incision caudally and finding no disc space at the level of S1–S2 vertebrae or by evidence of a bulging disc at L5–S1 level. The level was also be confirmed by direct visualization of crossing fibers of the thoracolumbar fascia, that usually begins below the level of L5–S1 disc space. In doubtful cases, the level was confirmed using fluoroscopy.

Statistical analysis

All statistical analysis was performed utilizing the Statistical Package for the Social Sciences version 16 (Chicago, Illinois, USA). Statistical significance value was accepted to be P < 0.05. The continuous variables were expressed in mean and standard deviation as descriptive statistics, whereas categorical variables were expressed in frequency and related percentage values. The Chi-square test was used to compare accuracies between different methods.

  Results Top

Mean and standard deviation values at assessment were; age, 45.56±11.075 years, 61.5% comprising male gender. The primary pathologies affecting the majority of patients were disc prolapsed with 41% at L4–L5 followed by 20.5% at L5–S1 level. Multilevel disc involvement was seen in 30.7% of the cases [Table 1].
Table 1: Demographic and spine-specific metrics of study cohort

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Twenty-two patients were subjected to preoperative localization X-rays to test the validity of the localization method described above. We analyzed these X-rays to see if there were any differences in the standing and supine postures of the patients. Around 73% of the patients were found to have exact localization of the marker near the L5–S1 space in the anteroposterior view taken in the standing position whereas it corresponded to 27% in the supine position [Figure 4]. On analyzing this topographical localization method by doing X-ray in standing position, the sensitivity came to be 93.3%, with a positive predictive value of 77.8% [Table 2]. Thus, the positioning of the patient during X-ray does affect the accuracy of localization. This difference was statistically significant with P = 0.003.
Figure 4: A graph representing the difference in supine and standing posture radiographs. Twenty-two patients who had X-rays in both postures; standing position more consistently represents L5–S1 level

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Table 2: Surface marking method more consistently reproducing results with X-ray in standing position

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[Table 3] shows the accuracy of spine level localization intraoperatively using the knowledge of topographical anatomy. Sixty-four percent of the patients had accurate L5–S1 spine level localization utilizing the above clinical palpatory methods, while 23% of the cases identified L5 spine [Figure 5]. On analyzing this result with that of X-rays in standing posture, the P value was 0.49, showing that this method is reproducing consistent and similar results both intraoperatively and X-rays in standing posture. However, when compared to X-ray in supine posture, the P value is 0.0057 (significant), indicating that consistent results are produced only intraoperatively.
Table 3: Intraoperative accuracy of spine level localization using knowledge of topographical anatomy

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Figure 5: A graph representing the accuracy of spine level localization intraoperatively using knowledge of topographical anatomy

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Intraoperatively, no difficulty was encountered in identifying the desired level by this localization method in these patients after their positioning in jackknife position, which facilitates the palpation of disc space more accurately with tip of the thumb.

Intraoperative fluoroscopy assistance was taken in 30.7% of the cases, especially those presenting disc prolapse at a higher level, likely L2–L3 level, and in some cases of multilevel disc prolapse for double confirmation of the level. Neither the disc space was missed nor was any intraoperative assistance of fluoroscopy required, especially when surgery was done for L4–L5/L5–S1 disc space. Irrespective of the preoperative localization result through X ray, on table surface marking after proper positioning over Wilson's framefollowing anesthesia was proved to be more accurate without any fail in overall cases.

  Discussion Top

The incidence of degenerative diseases of the spinal column is in an increasing trend due to the change in current lifestyle. Excess bike driving, jerky lifting of heavyweight, inadequate water intake, bad posture, smoking, and several other risk factors have led to an increase in the incidence of lumbar disc disease. An inadequate precaution after the degenerative process has started, predisposes disc prolapse and nerve root compression. Few of these patients require surgery due to persistent symptoms and/or progressive neurological deficits despite a trial of conservative treatment.

The method described here has been evolved from problems aroused while dealing with spinal cases in the beginning of the practice of lumbar spinal surgery by the author. Although intraoperative localization by fluoroscopy is well accepted and widely practiced, still there is every chance of wrong-level spine surgeries and unintended level of exposure. A systematic review by Devine et al. listed the incidence of wrong-level spine surgeries in the literature ranging from 0.09 to 4.5/10,000 surgeries.[9] Similarly, unintended level exposure has been reported to occur in anywhere from 1.3% to 15% of the cases.[10] Few recent studies have claimed that there is significant improvement in the incidence of these errors by adhering to the universal protocol; the following consensus derived best practice guideline or modifying intraoperative imaging techniques.[11],[12],[13] However, failure to obtain or difficulty in obtaining intraoperative radiographic images is one of the important causes of wrong-level spine surgery.[10],[14] Sometimes, the use of imaging technique is not feasible due to various reasons during surgery such as machinery problems, absence of technician, electric failure, and so on. Therefore, the search for an alternative reliable method of localizing the lower spinal level has led to finding out these regional anatomical surface markings which are helpful in localizing the L5–S1 space.

Although Tuffier's line is used widely as a landmark for rapid spinal level identification, its range varies from L3–L4 interspace to L5–S1 interspace.[1] Broadbent et al.,[2] Furness et al.,[3] and Kim et al.[15] have identified the correct spinal levels only in 29%, 30%, and 36% of the cases using the iliac crest as an anatomical landmark. The accuracy and reproducibility of this clinical palpation method are especially affected in the case of females and in patients with higher body mass index due to the difference in the thickness of subcutaneous adiposity and thoracolumbar musculature.[4] By using PSIS as a surface marker, O'Haire[16] suggested that the use of PSIS might reduce inter-examiner discrepancy more successfully compared to the use of the iliac crest method. Similar result was also shown by Kim et al.,[15] however, they found discrepancies between each examiner which varied up to 2.5 cm in few cases. They had a conclusion that PSIS palpation is prone to substantial inaccuracies as it is located at variable points on surface palpation. McGaugh et al.[17] suggested that PSIS reference line might be used to find S2 as a standard reference in both static and dynamic palpation in the lower lumbar spine.

Considering the above facts, here we have taken the dimple of Venus as a surface landmark. The sacral dimples that mark the position of the posterior superior iliac spine are level with the S2 vertebral spinous process.[6] It is more prominent in thin individuals and well palpated in obese persons just above the gluteal cleft. Clinically, it can be made prominent by asking the patient to stand and to contract the gluteal muscles. An imaginary line joining both dimples usually passes over the spinous process of the S2 vertebra. Thus far, the clinical application is concerned; they are frequently used as a guide to place the sacral pedicle screws in spinal surgery. The method described here is another new application of this dimple of Venus in spinal surgery.

This study revealed 63% accuracy of L5–S1 disc space localization followed by 23% for L5 spinous process intraoperatively. The inaccuracy was not detrimental to the patients as the level was subsequently confirmed either by extending the incision to identify no disc space available in S1–S2 vertebral level or in a few cases with the help of fluoroscopy. Preoperative localization X rays in standing posture more consistently identified L5–S1 disc space compared to supine posture (P = 0.003) indicating the position of the patients influencing the landmark on palpation. McGaugh et al.[17] reported that lack of axial load in the supine position could affect the relationship between the identified reference line and its corresponding spinous process. Similarly, other available evidences have demonstrated that change in position of legs affecting the amount of lumbar lordosis, thereby altering the reference line–spine relationship with lumbar flexion.[18],[19],[20] However, intraoperatively, no such difficulties were encountered on the OT table as jackknife positioning in lumbar disc surgery facilitates the palpation of disc space more consistently with the tip of the thumb after locating the described points.

Factors leading to increased accuracy or misidentification of pathology related to the spinal level were not considered in the present study. Tee et al. identified seven potential predictive variables of thoracolumbar spine localization such as age, sex, and adiposity.[21] Despite this limitation, this study is of value because in this method of localization we were able to identify L5–S1 disc space more accurately; simultaneously, it potentially decreases the radiation dose to both patients as well as to the surgical team intraoperatively. However, spinal level estimation by palpation could be influenced by the clinician's skill, patient's body size, and anatomical variations.

Even this short-term study with small sample size consistently shows the above-described method for identification of L5–S1 disc space, still generalization of the results for use in clinical practice is restricted and requires further investigation.

  Conclusion Top

The the surface marking method method described here accurately localizes L5-S1disc space in 64% cases and L5 spine in 23% cases. So from clinical stand point overall accuracy is 87%. Although it cannot replace the role of radiological assistance in localization during lumbar spine surgery still it is an important alternative for any spine surgeon in the beginning of their career and in certain specific situations.


The authors acknowledge Dr. Rashmi Das Q. No 3R/24, VIMSAR Doctor Colony, for providing [Figure 1] in the manuscript. [Figure 2] has been taken from the Osteology Section of the Anatomy Department of the institution with permission.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Render CA. The reproducibility of the iliac crest as a marker of lumbar spine level. Anaesthesia 1996;51:1070-1.  Back to cited text no. 1
Broadbent CR, Maxwell WB, Ferrie R, Wilson DJ, Gawne-Cain M, Russell R. Ability of anaesthetists to identify a marked lumbar interspace. Anaesthesia 2000;55:1122-6.  Back to cited text no. 2
Furness G, Reilly MP, Kuchi S. An evaluation of ultrasound imaging for identification of lumbar intervertebral level. Anaesthesia 2002;57:277-80.  Back to cited text no. 3
Chakraverty R, Pynsent P, Isaacs K. Which spinal levels are identified by palpation of the iliac crests and the posterior superior iliac spines? J Anat 2007;210:232-6.  Back to cited text no. 4
Atlíhan D, Bozkurt M, Turanlí S, Doğan M, Tekdemir I, Elhan A. Anatomy of the posterior iliac crest as a reference to sacral bar insertion. Clin Orthop Relat Res. 2004;(418):141-5. PMID: 15043105.  Back to cited text no. 5
Drake RL, Vogl AW, Mitchell AW. Grey's Anatomy for Students. 3rd ed. Philadelphia, Churchill Livingstone Elsevier, 2015 .p. 115.  Back to cited text no. 6
Stonelake PS, Burwell RG, Webb JK. Variation in vertebral levels of the vertebra prominens and sacral dimples in subjects with scoliosis. J Anat 1988;159:165-72.  Back to cited text no. 7
Miller SA, Mayer T, Cox R, Gatchel RJ. Reliability problems associated with the modified Schöber technique for true lumbar flexion measurement. Spine (Phila Pa 1976) 1992;17:345-8.  Back to cited text no. 8
Devine J, Chutkan N, Norvell DC, Dettori JR. Avoiding wrong site surgery: A systematic review. Spine (Phila Pa 1976) 2010;35:S28-36.  Back to cited text no. 9
Ammerman JM, Ammerman MD, Dambrosia J, Ammerman BJ. A prospective evaluation of the role for intraoperative x-ray in lumbar discectomy. Predictors of incorrect level exposure. Surg Neurol 2006;66:470-3.  Back to cited text no. 10
Grimm BD, Laxer EB, Blessinger BJ, Rhyne AL, Darden BV. Wrong-Level Spine Surgery. JBJS Rev 2014;2:e2.  Back to cited text no. 11
Vitale M, Minkara A, Matsumoto H, Albert T, Anderson R, Angevine P, et al. Building consensus: Development of best practice guidelines on wrong level surgery in spinal deformity. Spine Deform 2018;6:121-9.  Back to cited text no. 12
Patel A, Runner RP, Bellamy JT, Rhee JM. A reproducible and reliable localization technique for lumbar spine surgery that minimizes unintended-level exposure and wrong-level surgery. Spine J 2019;19:773-80.  Back to cited text no. 13
Hsiang J. Wrong-level surgery: A unique problem in spine surgery. Surg Neurol Int 2011;2:47.  Back to cited text no. 14
[PUBMED]  [Full text]  
Kim HW, Ko YJ, Rhee WI, Lee JS, Lim JE, Lee SJ, et al. Interexaminer reliability and accuracy of posterior superior iliac spine and iliac crest palpation for spinal level estimations. J Manipulative Physiol Ther 2007;30:386-9.  Back to cited text no. 15
O'Haire C, Gibbons P. Inter-examiner and intra-examiner agreement for assessing sacroiliac anatomical landmarks using palpation and observation: Pilot study. Man Ther 2000;5:13-20.  Back to cited text no. 16
McGaugh JM, Brismée JM, Dedrick GS, Jones EA, Sizer PS. Comparing the anatomical consistency of the posterior superior iliac spine to the iliac crest as reference landmarks for the lumbopelvic spine: A retrospective radiological study. Clin Anat 2007;20:819-25.  Back to cited text no. 17
Mody MG, Nourbakhsh A, Stahl DL, Gibbs M, Alfawareh M, Garges KJ. The prevalence of wrong level surgery among spine surgeons. Spine (Phila Pa 1976) 2008;33:194-8.  Back to cited text no. 18
Hirabayashi Y, Igarashi T, Suzuki H, Fukuda H, Saitoh K, Seo N. Mechanical effects of leg position on vertebral structures examined by magnetic resonance imaging. Reg Anesth Pain Med 2002;27:429-32.  Back to cited text no. 19
Jung CW, Bahk JH, Lee JH, Lim YJ. The tenth rib line as a new landmark of the lumbar vertebral level during spinal block. Anaesthesia 2004;59:359-63.  Back to cited text no. 20
Tee JW, Rutges J, Marion T, Street J, Paquette S, Ailon T, et al. Factors predictive of topographical accuracy in spine level localization. J Spine Surg 2017;3:23-30.  Back to cited text no. 21


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]

  [Table 1], [Table 2], [Table 3]


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